Whether you're writing fiction or non-fiction a character biography will help you remain focused on who it is you are moving through the book. Before I started putting my story into text I sat down and wrote several character biographies. You can see them at my website.
I'm a fiction writer so that is what I'll be focusing on. For non-fiction my guess is it can be very simple -- "A 42 year old working mother who doesn't mind cooking but hates grocery shopping." or "A 35 year old marketing manager with a bachelors degree and designs on the corner office." It's not a target market as much as it is a consistent frame of reference.
For fiction you have to create the depth. You want to bring a character to life. Readers should be able to identify with your character and feel like they can actually picture them. What better way to do that for yourself then to write down a character biography?
I know the pantsers out there will tell you they don't know what their character is like yet or that they don't do well with structure. Others will say that they have their character all mapped out in their head. The exercise of writing down details of a character helps you sort things out before you get into the weeds of your story. Does your character have siblings or are they an only child? Has your character ever broken a limb? What color hair and eyes do they have. Where are they from, have they ever moved? Much like a regular person the details are endless. It can be fun to get started and see where it leads.
In addition to being fun and sorting out details the character biography can also help break writers block. If you get stuck and don't know where to go next try the character bio. Maybe you'll find a mistake or something you forgot about the character. Maybe you need to add something juicy that you came up with while writing. Before you know it the character is all up in your head and there are a hundred different things you need to do with your story.
Also, lets face it, sometimes we have to take breaks from writing. If you have to go away from your story for a while then the character biography can help you get back into it later. Reading your notes can help you remember the plot and story arc. Reading what you have written can remind you of where you are in the story. The character biography can help you to remember who it is you are taking on the journey.
So where do you stand? Have you written character biographies or do you completely pants it?